1. Field of the Invention
The present invention relates to a method of producing a semiconductor device and, particularly, relates to a method of producing a semiconductor device mainly comprising a process which can be suitably adapted to the method of forming thin gate insulating film provided in MOSFETs (MOS field-effect transistors), TFTs (thin-film transistors), etc.
2. Description of the Prior Art
Heretofore, MOSFETs, TFTs, etc. are known as semiconductor devices constituted with silicon (hereinafter abbreviated as "Si") bases or Si thin films as substrates. For example, a dilute hydrofluoric acid solution or an ammonia-peroxide mixture solution is used for wet cleaning of the surface of Si before the formation of gate insulating film (Si oxide film: SiO.sub.2 film), in the process of producing such a semiconductor device.
The dilute hydrofluoric acid solution which is generally called "dilute fluoric acid solution" is an aqueous solution of hydrogen fluoride HF and hereinafter abbreviated as "dilute HF solution". The ammonia-peroxide mixture solution is a mixture solution of aqueous ammonia NH.sub.4 OH (or ammonia NH.sub.3) and hydrogen peroxide H.sub.2 O.sub.2 and hereinafter abbreviated as "APM (ammonia peroxide mixture) solution".
In the aforementioned conventional cleaning process, it is known that the surface condition of Si after cleaning is characterized (decided) by a solution used for final cleaning. That is, in the case where the dilute HF solution is used for final cleaning, a naturally oxidized film and a surface contamination layer characterized by HF cleaning are formed. In the case where the APM solution is used for final cleaning, a naturally oxidized film and a surface contamination layer characterized by APM cleaning are formed. In the case of APM cleaning, a perfectly clean Si surface cannot be formed so that not only a naturally oxidized film having a thickness of about 10 .ANG. or less remains but a surface having some characteristic exists therein.
In the conventional Si surface cleaning step in the method of producing a semiconductor device, not only naturally oxidized films respectively peculiar to slow etching solutions, such as an APM solution and a dilute HF solution, used as cleaning solutions are formed but surface contamination layers respectively depending on the naturally oxidized films exist. Therefore, the following problem is pointed out.
Though the formation of naturally oxidized film may be unavoidable, the final cleaning with the HF solution has a disadvantage in that the Si surface is hydrophobic and has the property that particles are easily deposited thereon, from the point of view of the microscopic surface form thereof. On the other hand, the cleaning with the APM solution has an advantage in that the Si surface is hydrophilic and has the property that particles are not easily deposited thereon, but the cleaning with the APM solution has a disadvantage in that the Si surface is contaminated with metals such as Fe and Al. In particular, the presence of such metal contamination is a conclusive barrier to the case where a gate oxide film is formed on the Si surface.
That is, such gate oxide film used in MOSFETs and MOS capacitors for memories has been thinned year by year. In particular, in the case of MOS capacitors, the thinning of the gate oxide film is unavoidable for the purpose of increasing the capacity per unit area. The present situation is that the thickness of the oxidized film of about 100 .ANG. is required. Because the aforementioned surface contamination easily induces failure in the insulating property of the oxidized film, a faultless thin oxidized film being free from surface contamination with metals is now required and demanded.